Even as a little kid I always did stupid daredevil stuff, but I think I really became self-destructive in my early teens by cutting. I haven't done any major self-harm for a few years now, but I do still get the urge. It's strange how cutting yourself can become an addiction that can still plague you years later.
I started when I was 14, and I started by burning myself. Lighters, stoves, you name it. Moved on to cutting when I was 16, and kept at it all until I was 20. I'm 27 now, and I'm still fighting off relapses.